(1) Field of the Invention
The present invention relates to transparent coatings for the dissipation of electrostatic charge, and more particularly to two-layer transparent coatings for the dissipation of electrostatic charge.
(2) Description of the Related Art
Many applications require coatings that dissipate electrostatic charge and many such coatings are known in the art. Most of these coatings contain some type of electrically conductive material, such as metal particles, and are opaque. However, when the surface that is to be protected from charge build up is transparent, such as a window, a computer screen, television screen, optical lens, lenses of eyeglasses, instrument covers, airplane canopies, or the like, the electrostatic dissipating (ESD) coating must also be transparent.
In the case of airplane canopies, it is not uncommon for several coatings to be applied, each designed for a different purpose. For example sputtered coatings of indium tin oxide (ITO), aluminum tin oxide, gold, or silver can act as a shield from electromagnetic interference, tie-coats of polymers can facilitate adhesion of non-compatible coatings, and hard coats provide the exterior surface of the canopy with durability and abrasion resistance.
Aircraft canopies must be able to dissipate electrostatic charge that is generated triboelectrically during flight. Since canopies are made of insulating plastics such as polycarbonate and polyacrylic, electrostatic charge builds up and may take minutes to hours to dissipate. The charge accumulated in flight is dangerous to pilots, maintenance personnel, and delicate on-board electronic equipment and may pose a potential explosion hazard. The deposition of transparent, sputtered ITO films onto the canopy surface has been the principal means for alleviating problems with charge build-up. However, sputtered films are inflexible and fragile as well as being susceptible to degradation by UV light, which is very intense in the upper atmosphere. Moreover, when the canopy has a metal-containing electromagnetic shielding layer covered with a non-conductive hard coat, arcing of electrostatic charge through the hard coat to the electromagnetic shielding layer can cause visible pinholes or pits in the coatings, which eventually compromise the optical qualities of the canopy and make replacement necessary. The aerospace industry needs a highly transparent ESD film that is easy to apply, flexible, adherent and stable to the extreme environments to which aircraft are exposed.
The prior art describes several ESD coatings that are designed for use over transparent substrates. Some of these coatings are designed to be applied in a single layer, for example, US 2003/0164427 A1 and US 2005/0230560 A1 describe coatings that contain carbon nanotubes. Other prior art ESD coatings can employ two or more layers, for example, US 2005/0042556 A1 describes a coating system employing a first layer comprising an intrinsically conductive polymer and a second layer consisting of a non-continuous layer of conductive silver. Coatings that are formed on transparent films are described in U.S. Pat. Nos. 6,146,753, 6,503,627, 7,014,912, and in US 2006/0029818 A1. In the first two of these, Niimi et al. describe a three part coating comprising a transparent substrate film on which is deposited a conducting layer, which can contain an intrinsically conductive polymer, over which is placed a hard coat having anisotropic electrical conductivity that is preferably obtained by constructing the layer with conductive fine particles having a diameter not less than ⅓ the thickness of the layer. In U.S. Pat. No. 7,014,912, and in US 2006/0029818 A1 Suzuki et al. describe a four part coating having a structure that is similar to that described above, but further having a top coat with a lower index of refraction than the hard coat to provide a low reflective antistatic hardcoat film.
It would be useful to provide a coating for an aircraft canopy, or indeed for any substrate on which a transparent ESD coating is useful, that could be applied directly to the substrate to be protected without the use of an intervening support film. It would be useful if the coating would be capable of effectively dissipating an electrostatic charge, but also had high visible light transmittance. In addition, it would be useful if such a coating exhibited good performance in the areas of coating adhesion to the substrate, flexibility, abrasion resistance, and chemical resistance.